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.Em Build directories
(or object directories, target directories, product directories, etc.) is
the concept that allows a developer to keep the source tree clean from
build products by asking the build system to place such build products
under a separate subtree.
.Pp
Most build systems today support build directories.  For example, the GNU
Automake/Autoconf build system exposes such concept when invoked as
follows:
.Bd -literal -offset indent
$ cd my-project-1.0
$ mkdir build
$ cd build
$ ../configure
$ make
.Ed
.Pp
Under such invocation, all the results of the build are left in the
.Pa my-project-1.0/build/
subdirectory while maintaining the contents of
.Pa my-project-1.0/
intact.
.Pp
Because build directories are an integral part of most build systems, and
because they are a tool that developers use frequently,
.Nm
supports build directories too.  This manifests in the form of
.Nm
being able to run tests from build directories while reading the (often
immutable) test suite definition from the source tree.
.Pp
One important property of build directories is that they follow (or need to
follow) the exact same layout as the source tree.  For example, consider
the following directory listings:
.Bd -literal -offset indent
src/Kyuafile
src/bin/ls/
src/bin/ls/Kyuafile
src/bin/ls/ls.c
src/bin/ls/ls_test.c
src/sbin/su/
src/sbin/su/Kyuafile
src/sbin/su/su.c
src/sbin/su/su_test.c

obj/bin/ls/
obj/bin/ls/ls*
obj/bin/ls/ls_test*
obj/sbin/su/
obj/sbin/su/su*
obj/sbin/su/su_test*
.Ed
.Pp
Note how the directory layout within
.Pa src/
matches that of
.Pa obj/ .
The
.Pa src/
directory contains only source files and the definition of the test suite
(the Kyuafiles), while the
.Pa obj/
directory contains only the binaries generated during a build.
.Pp
All commands that deal with the workspace support the
.Fl -build-root Ar path
option.  When this option is provided, the directory specified by the
option is considered to be the root of the build directory.  For example,
considering our previous fake tree layout, we could invoke
.Nm
as any of the following:
.Bd -literal -offset indent
$ kyua __COMMAND__ --kyuafile=src/Kyuafile --build-root=obj
$ cd src && kyua __COMMAND__ --build-root=../obj
.Ed
